Color xerography



Nov. 29, 1960 R. M. SCHAFFERT COLOR XEROGRAPHY Filed May 2, 1956 SECOND"PRIMARY COLOR F! RST PRIMARY COLO INVENifaR. ROLAND M- SCHAFFERT Y BFNM W ATTQlfZM'y United States Patent COLOR XEROGRAPHY Roland M.Schaffert, Columbus, Ohio, assignor, by mesne assignments, to HaloidXerox Inc., Rochester, N.Y., a corporation of New York Filed May 2,1956, Ser. No. 582,107

2 Claims. (Cl. 96-1) This invention relates in general to xerognaphy andin particular to the xerographic reproduction of color images.

In the art of xerography it is usual to record and reproduce byelectrical means a black and white copy of an original scene, documentor the like. Such xerographic reproduction has been brought intocommercial use for the making of line copy images and, to a certainextent, for continuous tone reproduction. At the present time, however,there is no system commercially available for producing coloredxero-graphic prints. It is, therefore, an object of the presentinvention to provide means, method and apparatus for the production offull colored xerographic prints.

It is a further object of the invention to provide means, methods andapparatus for two or more color xerognaphic reproduction and for theaddition of other colors thereto.

It is an additional object of the invention to provide a new method forreproduction of xerographic images corresponding to two primary colorsof light.

It is a still further object of the invention to provide new colorxerographic apparatus comprising a light sensitive xerographic membercharacterized by having two separate sensitive xerographic layers eachselectively sensitive to exposure by means of a different primary color.

It is a still further object of the invention to provide new means,methods and apparatus for color xerography whereby a xerographic printor reproduction in two colors can be produced with a single exposure tolight of mixed primary colors.

In accordance with the present invention, a plurality of photoconductivelayers of selected conductive sensitivity to light of different primarycolors are simultaneously exposed to a projected light image of mixedprimary colors. For example, a plurality of photoconductive layers maybe superposed, one on top of the other in laminated configuration, and alight image of mixed primary colors may be projected onto the array ofphotoconductors from one side thereof so as to pass selectively throughthe first of the photoconductors in reaching the last of thephotoconductors. An electric field is applied through thephotoconductive layers while they are exposed in this manner so as toproduce a plurality of develop able xerographic latent images which maybe developed and superposed to form a two-color or full colorxerographic reproduction.

The general scope and nature of the invention having been set forth, theinvention is further illustrated in the following specification and inthe drawings in which:

Fig. 1 is a diagrammatic view of a two-color xerographic plateillustrating its exposure to light of mixed primary colors;

Fig. 2 is a diagrammatic view of a xerographic plate adapted to record athird color.

In Fig. 1 there is illustrated a color xerographic sensitive membergenerally designated comprising a transparent conductive support base 11having on its one surface thereof a photoconductive insulating coatingor layer 2,962,375 Patented Nov. 29, 1960 ICQ 12 characterized bysensitivity to a first primary color and on its opposite surface aphotoconductive insulating layer or coating 13 characterized bysensitivity to a second primary color. As shown in the figure, themember can be exposed to light of mixed primary color from the side ofthe first layer 12 whereby light of a first primary color is incident onand conductively activates the layer 12 as illustrated at point 15, anda light of a second primary color is transmitted through said firstphotoconductive layer 12 and through the support 11 to be incident onand conductively activate layer 13 as illustrated at point 16.

The first photoconductive layer 12 comprises a thin layer of a materialwhich is normally insulating and is conductively sensitive to light ofone primary color to the substantial exclusion of sensitivity to asecond primary color. It is characterized by the ability, in the absenceof radiation of this first primary color, to accept and retain on itssurface a usable xerographic charge potential and to selectivelydissipate this charge potential upon the exposure of active radiation ofthe first primary color. This layer may comprise any photoconductiveinsulating material meeting these general characteristics. It has beenfound, for example, that a layer comprising zinc oxide particlesdispersed in a solid insulating transparent binder such as a siliconeresin, as disclosed in copending application Serial No. 311,546 by A. E.Middleton and D. C. Reynolds is characterized by being conductivelysensitive to blue light and substantially insensitive to light of otherprimary colors. Thus, according to one embodiment of the invention,layer 12 may comprise a normally insulating material conductivelysensitive to blue light and substantially insensitive to red and greenlight, such as, for example, a binder coating of zinc oxide, or of Zincselenide, zinc sulfide, mixed zinc sulfide-cadmium sulfide, or titaniumdioxide. Other materials may be used such as, for example, bindercoatings of other photoactive oxides, sulfides, silicates and selenidesof cadmium, zinc, calcium, magnesium and the like, selected from thosethat are also characterized by appropriate sensitivity to a singleprimary color. Layer 12 may also be a vacuum evaporated coating ofvitreous cadmium sulfide, which is a transparent yellow materialconductively sensitive t blue light.

The second photoconductive layer 13 comprises a normally insulatinglayer conductively sensitive to light of a second primary color andoptionally substantially insensitive to light of the remaining or thirdprimary color. Thus, for example, if the first layer is conductivelysensitive to a primary color other than blue, this second layer 13 maybe a blue sensitive layer such 'as, for example, the zinc oxide binderlayer mentioned hereinbefore. If, on the other hand, the first layer 12comprises a zinc oxide binder layer as illustrated, then the secondlayer 13 is conductively sensitive to a second primary color, such asgreen or red. lllustratively, a layer of vitreous or amorphous selenium,preferably deposited on a conductive surface under vacuum and at .acontrolled temperature, is characterized by substantial sensitivity togreen light and is substantially insensitive to red light. For example,a layer of vitreous selenium about 10 to- 20 microns thick deposited invacuum at a substrate temperature of about 40 to 60 C. is conductivelysensitive to green light and virtually insensitive to red light; thislayer is a preferred companion to the zinc oxide binder layer. Othersuitable layers include binder coatings of mixed cadmium selenide-zincselenide, mixed lead oxide-zinc oxide, and mixed mercuric sulfide-zincoxide.

The conductive support 11, on which layers 12 and '13 are disposed, isatransparent conductive material such as, for example, conductive glassor transparent plastic having a conductive coating on each surfacethereof, or

a composite member comprising two sheets of glass or plastic, each witha single conductive coating on its outwardly facing surface. In apreferred embodiment of :the invention the glass may be tinted to absorbthe primary-color of sensitivity of layer 12. Thus, for example, iflayer 12 comprises a zinc oxide binder coating then 'the'glass member 11desirably will be tinted so as to absorb substantially all the strayblue radiation that might be passed through photoconductive layer 12 andin this case layer 13 may permissibly be sensitive to blue as well asgreen light.

In operation of the device illustrated hereinbefore an electrostaticcharge of about 150 volts is placed on each of layers 12 and 13 bysuitable means such as for example the charging methods illustrated inCarlson Patent 2,588,599 wherein an ion source such as a coronadischarge electrode may be passed across a surface to be charged. Thevoltage on the photoconductive layers as well as the polarity of thepotential may be selected In one In this case the binder coating of zincoxide is principally sensitive to blue light and is most satisfactorilyoperable when it has been sensitized by charging to make it of negativepolarity. A selenium coating, on the other 'hand, is sensitive togreen'light and is most satisfactorily operable when sensitized bycharging to positive polarity. Thus, in accordance with this operation,layer 12 is charged'prior to exposure to a negative potential of 150volts and layer 13 is charged prior to exposure to a positive potentialof 150 volts. The plate as thus charged exposed to a light image ofmixed primary colors projected on to the plate from the side of a zincoxide binder layer 12. The blue light thus is incident on layer 12 andis selectively absorbed by the layer causing it to be conductive inresponse to photon excitation whereby the negative charge on the layeris selectively dissipated by photoconductivity in accordance with and inproportion with the incident blue light. The remaining light image,including the primary color green is transmitted through layer 12 andthrough transparent support base 11 whereupon it becomes incident onlayer 13. At layer 13- at least the green light incident on the layersimilarly selectively dissipates the charge on this layer. Optionally,and as is the case in this illustrated embodiment of the invention,layer 13 may be insensitive to the third primary color whereby red lightis immaterial in selective dissipation of charge on the layer. Ifdesired, a suitable filter or dye may be incorporated as part of supportbase 11 to filter out the undesired or red component of the light orsuch a filter may be employed between the image source and the plate 10.Thus, the transparent plate 11 may be green so as to transmitsubstantially exclusively the primary color to which layer 13 issensitive or a cyan filter may be employed between the image source andplate to block the passage of red light. In this latter case base 11 mayalso contain a blue absorbing dye or filter. If transparent plate 11 isof composite construction a sheet of color filter material may beinterposed between the elements thereof.

In addition to their function of completely blocking undesired primarycolors either or both of the above described filter means may be chosento partially absorb selected wavelengths to which the photosensitivelayers are unduly sensitive thereby achieving more uniform response ofthe layers to the various wave-lengths of light comprising the primarycolors to which the layers are intended to respond.

Upon completion of charging and exposing the blue electrostatic image onlayer 12 and the green electrostatic image on layer '13'may be developedby suitable means. For example, a powder of color complementary to theprimary light of exposure and charged to polarity opposite to thepolarity to that of the photosensitive layer may be passed across thesurface of the layer to adhere selectively to the charged areas inproportion to the charge. In the case of-layer 12 the blue image isdeveloped with a yellow colored powder and the green image developedwith a magenta powder. This development can be carried out by anysuitable development methods, such as, for example, methods described inCarlson 2,297,691 and Carlson 2,221,776 or the methods of Wise2,618,552. According to a preferred embodiment of xerographicdevelopment operations, the xerographic latent images were developed byplacing them in a development zone with a closely spaced conductivesurface or development electrode about 4 -inch removed from the imagesurface and passing between the two surfaces a cloud or gas suspensionof finely divided charged powder particles as shown in Landrigan et al.2,725,304. Where it is desired to develop a xerographic image bydeposition of charged particles on the charged plate areas, this may beaccomplished by maintaining the backing member of thexerographic plateand the development electrode at substantially the same electricpotential. Where, on the other hand, it is desired to produce a reversal-print in which developer particles are deposited "on therelatively uncharged areas and the charged areas aremaintainedsubstantially free from deposit, this may be accomplished by biasing thedevelopment electrode at "charged areas formed in the same manner froman originally positively charged surface. These areas, therefore, aredeveloped with negatively charged magenta powder particles which areintroduced between the image layer and the development electrode. Thereare formed in this manner developed images of colors complementary tothe two primary colors. These images are therefore transferred to asingle support base, in register, to form a two-color xerographic print.

Image transfer may be accomplished by laying an image support layer,such as a sheet of paper, on the photosensitive coating, applying to theback of the layer as by a corona discharge device an electrostaticcharge of the same sign as was used to charge the photosensitive coatingand stripping the image support layer with the image now adhering to itfrom the photosensitive coating. Since the two images on the xerographicplate are in mirror image relation to each other they must either betransferred to opposite sides of a transparent support layer or one ofthem must be reversed before it can be superimposed upon the other. Thismay be done by transferring one image to a sheet of paper or the like bythe method described above and transferring the second image to anintermediate support such as another sheet of paper or preferably asheet of transparent plastic such as celluloseacetate. The side of theintermediate support bearing the second image is then laid on the firstimage and an electrostatic charge of sign opposite to that first used isapplied'to the back of the intermediate support, driving the secondimage onto the first. The intermediate support is then removed. leavingthe two images super posed in register. The resulting image may besprayed with clear lacquer to prevent smudging.

In Fig. 2 is illustrated :a further photosensitive memher by means ofwhich the third primary col-or may be added toxthe xerographic systemhereinbefore disclosed.

"Shown'in Fig. 2 is ase'cond xerograp'h'ic plate generally designated 24comprising a conductive support base 25 and a photoconductive insulatingcoating or layer 26 sensitive to at least the third primary color. Thus,in the event that the plate illustrated in Fig. 1 is sensitive to thetwo primary colors, blue and green, then layer 26 is sensitive to thethird primary color, red. P hotoconductive materials which possess redsensitivity include, for example, silicon, tellurium, tellurium-seleniummixtures, and many photoactive materials of the so-called phosphorclasses, including, for example, certain of the photoactive oxides,sulfides, selenides and silicates of cadmium, zinc, calcium, and thelike. For example, in a preferred embodiment of the invention axerograplhic plate was prepared by first evaporating onto a clean brasssurface a 50 micron layer of substantially pure selenium at a backingplate temperature of about 60 0., followed by evaporating a 2 micronlayer containing substantially 90 percent selenium and percenttellurium, directly on the freshly deposited selenium layer withoutbreaking the vacuum. The xerographic plate thus prepared was highlysensitive to red light.

This plate is charged to a positive potential or 150 volts and exposedthrough a red filter to the same light image of mixed colors as plate10. In this manner only the third or red primary color is incident onthe xerographic plate 24. There is thus formed a xerographic orelectrostatic latent image corresponding to this third primary color andthis image is developed in the same manner with developer materialcomplementary to the primary color. Thus, in the case of the red image acyan colored developer material is employed. 'Ilhis cyan image is thentransferred to the support base in register with the images alreadythereon.

While there are innumerable combinations of wavelengths corresponding toany particular sensation of color, the terms blue, green and red havebeen used in accordance with the usual terminology of color photographyto mean short, intermediate, and long wavelengths of visible light,respectively, and blue, green, and red filters or dyes are those whichtransmit the indicated portions of the spectrum to the exclusion ofothers. Similarly, yellow, magenta, and cyan dies, filters, or pigmentsare those which selectively absorb the short, intermediate, and longwavelength portions respectively of the visible spectrum.

It will be obvious that numerous modifications may be made in thepresent invention without departing from the present scope thereof. Forexample, xerograpliic development may be carried out according tosubtractive color principles with the use of colors complementary to thethree primary colors or alternatively additive develop ment may beemployed with developers of the primary colors themselves. Thus, forexample, added brilliance may be achieved through the use of a coloredglass developer or the like.

In like manner, the photoconductors may be first charged to a desiredelectric potential and polarity and then placed in position asillustrated in Fig. 1. Alternatively, any or all of the photoconductorsmay be placed in position in an uncharged condition and a field appliedthereto by means of an electric potential maintained between supportmember 11 and an adjacent plane electrode which must be transparent ifit is to be used adjacent to layer 12. These and other modifications andvariations will be within the scope of the present invention.

What is claimed is:

1. The process of color xenography comprising projecting a light imageconsisting of a pattern of light and shadow of at least a first and asecond primary color onto and through a first photoconductive insulatinglayer adllerent on a substantially transparent conductive member andonto a second photoconductive insulating layer adlierent on the oppositeside of said member while maintaining an electric field through saidlayers to form an electrostatic latent image thereon, said firstphotoconductive layer being conductively sensitive to light of at leastone first primary color and both substantially transparent to andsubstantially insensitive to light of at least one second primary color,said second photoconductive insulating layer being conductivelysensitive to light of at least said second primary color, one of saidphotoconductive layers consisting essentially of a dispersion in atransparent insulating binder of a material selected from the groupconsisting of zinc oxide, cadmium sulfide, mixed zinc sulfide cadmiumsulfide, zinc sulfide, zinc selenide and titanium dioxide, and the otherof said photoconductive layers consisting essentially of a materialselected from the group consisting of vitreous selenium, a dispersion ofmixed cadmium selenide-zinc selenium in a transparent insulating binder,a dispersion of mixed lead oxidezinc oxide in a transparent insulatingbinder, and a dispersion of mixed mercuric sulfide-zinc oxide in atransparent insulating binder, developing said latent images withcolored finely divided particles of colors complementary to said firstand second primary colors respectively, and super-posing the developedcolor image for the first primary color with the developed image for thesecond primary color on a common support layer.

2. The process of color xerogr-aphy comprising projecting a light imageconsisting of a pattern of light and shadow of at least a first and asecond primary color onto and through a first photoconductive insulatinglayer adherent on a substantially transparent conductive member and ontoa second photoconductive insulating layer adherent on the opposite sideof said member while maintaining an electric field through said layersto form an electrostatic latent image thereon, said firstphotoconductive layer being conductively sensitive to light of at leastone first primary color and both substantially transparent to andsubstantially insensitive to light of at least one second primary color,said second photoconductive insulating layer being conductivelysensitive to light of at least said second primary color, saidtransparent conductive member being substantially transparent to theprimary color of photosensitivity of said second photoconductiveinsulating layer and substantially opaque to the primary color ofphotosensitivity of said first photoconductive insulating layer, one ofsaid photoconductive insulating layers comprising vitreous selenium andthe other of said photoconductive insulating layers comprising zincoxide in a transparent insulating binder, developing said latent imageswith colored finely divided particles of colors complementary to saidfirst and second primary colors respectively, and superposing thedeveloped color image for the first primary color with the developedimage for the second primary color on a common support layer.

References Cited in the file of this patent UNITED STATES PATENTS1,191,941 Brewster July 25, 1916 1,986,280 Mueller Jan. 1, 19352,297,691 Carlson Oct. 6, 1942 2,320,693 Yauck et al. June 1, 19432,654,853 Weimer Oct. 6, 1953 2,739,243 Sheldon Mar. 20, 1956 2,803,542Ullrich Aug. 20, 1957 2,808,328 Jacob Oct. 1, 1957 2,824,986 Rome Feb.25, 1958 2,844,493 Schlosser July 22, 1958 2,844,543 Fotland July 22,1958 FOREIGN PATENTS 201,301 Australia Apr. 21, 1955 201,416 AustraliaApr. 13, 1956 OTHER REFERENCES Mees: Photography, 1937, pgs.19'1 and192, Macmillan Co. N.Y.

R.C.A. Review, December 1954, pp. 475-477, 96-1 lit.

1. THE PROCESS OF COLOR XEROGRAPHY COMPRISING PROJECTING A LIGHT IMAGECONSISTING OF A PATTERN OF LIGHT AND SHADOW OF AT LEAST A FIRST AND ASECOND PRIMARY COLOR ONTO AND THROUGH A FIRST PHOTOCONDUCTIVE INUSLATINGLAYER ADHERENT ON A SUBSTANTIALLY TRANSPARENT CONDUCTIVE MEMBER AND ONTOA SECOND PHOTOCONDUCTIVE INSULATING LAYER ADHERENT ON THE OPPOSITE SIDEOF SAID MEMBER WHILE MAINTAINING AN ELECTRIC FIELD THROUGH SID LAYERS TOFORM AN ELETROSTATIC LATENT IMAGE THEREON, SAID FIRST PHOTOCONDUCTIVELAYER BEING CONDUCTIVELY SENSITIVE TO LIGHT OF AT LEAST ONE FIRSTPRIMARY COLOR, AND BOTH SUBSTANTIALLY TRANSPARENT TO AND SUBSTANTIALLYINSENSITIVE TO LIGHT OF AT LEAST ONE SECOND PRIMARY COLOR, SAID SECONDPHOTOCONDUCTIVE INSULATING LAYER BEING CONDUCTIVELY SENSITIVE TO LIGHTOF AT LEAST SAID SECOND PRIMARY COLOR, ONE OF SAID PHOTOCONDUCTIVELAYERS CONSISTING ESSENTIALLY OF A DISPERSION IN A TRANSPARENTINSULATING BINDER OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OFZINC OXIDE, CADMIUM SULFIDE, MIXED ZINC, SULFIDE-CADUMIUM SULFIDE, ZINCSULFIDE, ZINC SELENIDE AND TITANIUM DIOXIDE, AND THE OTHER OF SAIDPHOTOCONDUCTIVE LAYERS CONSISTING ESSENTIALLY OF A MATERIAL SELECTEDFROM THE GROUP CONSISTING OF VITREOUS SELENIUM, A DISPERSION OF MIXEDCADMIUM SELENIDE-ZINC SELENIUM IN A TRANSPARENT INSULATING BINDER, ADISPERSION OF MIXED LEAD OXIDEZINC OXIDE IN A TRANSPARENT INSULATINGBINDER, AND A DISPERSION OF MIXED MERCURIC SULFIDE-ZINC OXIDE IN ATRANSPAREN INSULATING BINDER, DEVELOPING SAID LATENT IMAGES WITH COLOREDFINELY DIVIDED PARTICLES OF COLOR COMPLEMENTARY TO SAID FIRST AND SECONDPRIMARY COLORS RESPECTIVELY, AND SUPERPOSING THE DEVELOPED COLOR IMAGEFOR THE FIRST PRIMARY COLOR WITH THE DEVELOPED IMAGE FOR THE SECONDPRIMARY COLOR ON A COMMON SUPPORT LAYER.